1. Field of the Invention
The present invention relates to neutron radiation detectors. More specifically, the present invention relates to a method for calibrating neutron multiplicity counters.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Correlated neutron counting using shift register analysis is an important and long established tool for the detection and quantification of fission events in waste and safeguards. A crucial step in the manipulation of the resultant multiplicity histograms to correlated event rate is the application of correction factors for dead-time losses. Dead time correction can limit the accuracy especially when highly efficient counters are used to measure items of high neutron output where a high proportion of the primary neutrons are random (e.g. (α, n) events).
There is presently no complete treatment for this problem. Advances in instrumentation design to lessen the effects are an important active area of interest. However, these approaches also require similar correction, albeit of lesser magnitude, until a higher event rate is reached. Thus dead time correction remains a vital area of importance in neutron counting.
At present the most popular approach to making allowance for dead time losses is based on the approximation developed by Nikolai Dytlewski. In this scheme, the losses in the signal triggered rates can be compensated for by using histogram multipliers which are functions of a single free parameter—the dead-time parameter, τ. In practice however some additional empirical adjustments are often required. The issue then becomes how to best find the optimum value of the dead-time parameter.
The traditional method of applying dead-time corrections to passive neutron coincidence counting based on shift register electronics is typified by the treatment described by Menlove and Swansen for the case of the HLNCC-II. The singles rate is corrected using the exponential of a quadratic through the origin of the observed Singles rate. The Reals rate is only a function of the Singles rate and is set equal to the Singles correction factor raised to the fourth power.
A variation used by some is to replace the quadratic by the corrected Singles rate itself so that the expression is transcendental but for all practical purposes can be evaluated using a low order (e.g. 7) nested set of exponentials. The multiplier for the Reals correction is also left as a free parameter although the expectation is that it be close to four times that of the Singles dead-time parameter. This approach is set out in Croft and Yates. However, for multiplicity counting the correction for Triples is more complex and the approximate approach described by Dytlewski is the most widely used treatment.
Accordingly, a need exists for a more efficient calibration method that allows for field calibrations and verifications of the multiplicity dead time parameter. Further, a need exists for a calibration method that reduces the required number of reference sources and lengthy measurement times. Further, a need exists for a simplified calibration method that allows for readily repeatable calibrations to be performed over the life of a counter. Finally, a need exists for a method for calibration that does not require use of costly NIST certified source. The present invention satisfies these needs and others as will be explained in the following description.